®
Usage of helium in
Hydropnuematic accumulators
Updated September 2007
accumulators.com
1175 Brittmoore Rd. Houston, TX 77043 USA
voice: 713-465-0202 fax: 713-468-1618
e-mail: info@accumulators.com
Over the last decade there have been numerous discussions regarding the usage of helium (He) in replacement of
nitrogen (N2) in hydra-pneumatic accumulators. In the mid ‘90s the concern was that it appeared N2 was
liquefying or solidifying under extreme pressure and low temperature. Our subsequent investigation found that this
was not the case, but rather it was likely caused by impurities (such as water vapor) in the gas. We found that the
type of gas was not significant but rather the grade of the gas is significant. The cost of Research Grade gases along
with several other factors made the use of helium a dead issue.
In the intervening years we conducted numerous hydraulic, pressure and hyperbolic tests and found little difference
in the short-run performance of helium vs. nitrogen. We did however find some limited evidence (as predicted in
our original investigation) of helium permeation of the rubber and leakage of gas.
In recent months there has been renewed discussion of the use of helium, this time in order to maximize the
potential fluid volume of existing accumulator systems. We have now completed our latest research and have found
that helium is an even less desirable medium than our prior report had indicated.
 Our original report found that compressibility of He vs. N2 was primarily on a molecular level, but on a
physical level there was little difference. Our new investigation has confirmed our original conclusion.
We have found that the Sport Diving industry has collected a great deal of information regarding gas
compressibility in deep-sea applications. At 8000 feet the difference is less than 1%. At 12,000 about
6%. This is hardly a significant improvement, considering all the other problems with helium.
 In the 1998 report we expressed concern that permeation of the bladder might cause explosive
decompression. This remains a serious and dangerous problem as accumulators are subjected to
even greater depths and higher pressures.
 Covered in the first report were our concerns with gas leakage, and the high cost and limited
availability of helium. While helium is somewhat more available than 10 years ago, the other
concerns remain.
During our recent investigation we uncovered a number of factors that had not been previously considered.
 Helium is one of Nature’s most conductive elements. The thermal expansion and reduction of helium is
far more volatile than N2. While we could find no test data, the concern is that a rapid rise in
temperature (such as when a subsea unit goes from depth to surface) could lead to dangerous Explosive
Thermal Expansion. On the flip side, when going from surface to depth, the falling temperature could
reduce the initial precharge too quickly and combined with the pressure at depth, could cause extreme
stress on the shell exterior.
 Helium gas has several unique properties. Among these is a negative Joule-Thompson Coefficient.
Most users of accumulators are familiar with the fact that Nitrogen chills as it expands, so when
precharging, care must be taken not to “freeze” the bladder. With helium, the opposite is true. Care
must be taken not to “melt” the bladder. However of bigger concern is that during operation as the fluid
is discharged, the gas will heat up quickly, thus adding to the Explosive Thermal Expansion problem
already discussed.
 Lastly, of some concern, is that the Rockwell Corporation has patented the use of helium in
accumulators. Users should contact their legal departments to determine their usage standing.
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Thursday, November 19, 1998
Usage of helium In Hydropnuematic accumulators
Dear Subsea Customer;
A number of our customers have been discussing the usage of helium in their subsea control
systems. Several engineering and consulting companies have also approached us. A few
companies have implied that this may be proprietary or confidential information. Our position
will be that the usage of helium is neither confidential nor proprietary, but that the products,
procedures or processes that each company develops may be so. In any case, per our
established policy, Accumulators, Inc. will not share any confidential or proprietary information
with others.
Our preliminary review of the usage of helium as a precharge gas has not been encouraging.
We have found several areas of concern that you, as the developer, should be aware of:
1. Helium is a much smaller molecule (molecular wt. 4 vs. N2, which is 28). This may
exacerbate a condition called permeability (the rate at which gases pass through a
membrane, such as a bladder or o-ring seal). Our tests show that helium is 9 times
more permeable than nitrogen. What this may mean is that the gas will “leak” from the
bladder into the fluid at this higher rate. The same problem will be found, (to a higher
degree), in a piston, float or diaphragm accumulator. In addition, the smaller molecule
will more readily entrain (mix) with the fluid.
2. The smaller helium molecule will also be more likely to leak through metal seals. All
high-pressure gas valves have metal-to-metal seals. It is possible that a new (read
expensive) valve could be developed. However, keep in mind, leakage tests are
always conducted with helium, since it’s the most difficult to seal!
3. Helium is a much more expensive gas, costing 400% more than Nitrogen. (We have
discussed the usage of helium with the industrial gas suppliers. They are very
anxious for this change to take place!) . It is also less readily available, and virtually
unavailable in many parts of the world.
4. We understand that one reason Helium is being investigated is that it appears that
nitrogen may be freezing at extreme depths. Nitrogen does not freeze naturally,
however water and other impurities may do so. The commonly used industrial grade
gases whether Helium or Nitrogen are 99.9% pure. To virtually eliminate these
impurities, one must use Research grade gas, 99.9999 % pure, which is 3000% more
expensive. In addition you will need to develop precharging procedures that have
permanently sealed connections, to keep out atmospheric impurities. In the rough
offshore environment this will be difficult if not impossible. The use of dryers & filters
will also be necessary. (Dryers & filters alone may accomplish the same task.)
5. Another concern that has been expressed to us is the mistaken belief that N2
liquefies under extreme pressure. Actually one must reduce N2 to –320F to reach
this condition.
6. One additional concept has been put forward. Since Helium is a smaller molecule, it
will compress much more than Nitrogen and consequently allow for more fluid in the
accumulator. This is true at the molecular level, however on the physical level this will
not be the case. Under the Ideal gas law (PV=nRT), n= number of molecules, P is
2
pressure, V is volume, T is temperature and R is the universal gas constant. So, on
a molecular level, given the same number of molecules, helium would squeeze into a
smaller volume than nitrogen, at the same temperature and pressure. However in the
application of an accumulator the beginning volume is fixed (such as a 15 gallon
accumulator). Consequently the starting point (the gas precharge) is the same for any
gas. It just takes more molecules of helium to start with. We then turn to Boyles Law
of gases P1V1=P2V2 which is not a finite law, but a close physical approximation for
both He and N2. Under this observable rule, in a fixed volume vessel, the ratio of gas
volume to fluid volume is proportionate to the beginning precharge pressure and the
maximum fluid working pressure. The type of gas does not significantly change the
result. (Some hydrocarbon gases do act irregularly).
7. Another concern is Explosive Decompression. At extreme depth and pressure,
gases, which permeate the seals and bladders, tend to seek out normal microscopic
level cavities. When the pressure is released too quickly, the gas expands and can
deform the elastomer. The problem is much worse with helium.
8. Finally, there has been very little practical usage of helium in bladder accumulators.
We do not, at this time, have any useful data, nor have we been able to discover any
information as to the performance of this gas. The opinion of the most experienced
people is that the difficulties will far exceed the benefits, if in fact there are any
benefits.
This memo is not meant to dissuade your investigation of the use of helium, it is only
meant to make you aware of several of the potential concerns that we have uncovered so
far. You will want to investigate these and other currently unknown problems thoroughly,
to your own satisfaction.
We want to assure you that Accumulators, Inc. stands ready to provide you with
assistance in this, or any other, product’s development. We look forward to hearing from
you in the future.
Jeffry A. Schneider
President
Accumulators, Inc.
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